With the recent development of highly-advanced information technology, the transmission of larger-volume information through a network such as the Internet has been demanded. An optical communication system and an optical information processing system have attracted attention as communication systems capable of transmitting large-volume information at high-speed.
Recently, a larger-capacity optical communication system and a larger-capacity optical information processing system have been developed in response to the above-described demand. In these optical communication system and optical information processing system, in order to perform a high-speed operation and to have a longer transmission distance, it is needed to use optical signals having less optical frequency chirping so as to make the effect of fiber dispersion, which is a cause of waveform deterioration, smaller. For this reason, recently, it has been a mainstream to use a configuration in which a light source at a DC operation and an external modulator are combined, so as to generate optical signals.
And now, a waveguide-type optical control device is one of key elements of the high-speed optical communication system and the optical information processing system. Of the waveguide-type optical control devices, an optical modulator is an essential device for converting an electric signal of a voice, an image, or the like into a level of light. The optical modulator is briefly classified into one using a dielectric such as LiNbO3 (LN) and one using a semiconductor such as InP or GaAs.
As a typical external modulator (optical modulator), a LiNbO3 (LN) modulator using a dielectric such as LiNbo3 (LN) has been widely used today. This operates with an electro-optic effect that a refractive index of a medium is changed by applying an electric field of direct current or an electric field with a frequency sufficiently lower than that of light.
The optical modulators using such an electro-optic effect include a phase modulator which modulates a phase of light by changing a refractive index of a dielectric having an electro-optic effect, and also include a light intensity modulator that is composed of phase modulator and a Mach-Zehnder interferometer. The Mach-Zehnder-type optical modulator can remove frequency chirping in principle, and therefore is a suitable modulator for ultra high-speed and long-distance communications.
However, since the conventional LN modulator is relatively long element length, the module size becomes large in addition a high driving voltage in a range from 3 to 5V is required. Furthermore, since a driving condition changes due to a DC drift (direct voltage drift) or a temperature drift, a control circuit is needed for a stable operation. In other words, there has been a problem in that a mechanism for controlling the driving condition is needed due to that change of the driving condition.
On the other hand, the optical modulators using a semiconductor include the following two typical modulators. One is an electroabsorption-type optical modulator (EA modulator) that uses an absorption edge shift toward a long wavelength side when an electric field is applied, such as a Franz-Keldysh Effect in a bulk semiconductor or a Quantum Confined Stark Effect (QCSE) in a multi-quantum well structure. The other one is an electro-optic modulator (EO modulator) that uses the electro-optic effect (Pockels effect) of changing a refractive index by applying an electric field.
The electroabsorption-type optical modulator (EA modulator) has attractive features such as small chip size, low-power consumption, and no DC drift that is seen in a LiNbO3 modulator. Consequently, it is expected to be promising. However, the electroabsorption-type optical modulator (EA modulator) has frequency chirping, and this chirping causes the deterioration of waveform occurs after optical fiber transmission. In other words, the frequency chirping causes the optical signal spectrum after the modulation to be wider than the optical signal spectrum before the modulation. When this optical signal with the widened spectrum is transmitted through the optical fiber, the deterioration of waveform due to the effect of dispersion of an optical fiber medium occurs, which results in causing a further unfavorable effect. This phenomenon of the waveform deterioration becomes more serious as a bit rate is higher and a transmission distance is longer.
On the other hand, as for the electro-optic modulator (EO modulator), a phase modulator that modulates a phase of light by changing a refractive index, and a Mach-Zehnder modulator that modulates light intensity with the formation of a Mach-Zehnder interferometer by combining it with the phase modulator, are in practical use. In today's optical communications, a signal is transmitted based on a level of the light intensity, and therefore the Mach-Zehnder modulator performing intensity modulation is mainly used. This Mach-Zehnder modulator can completely remove frequency chirping in principle, and therefore is greatly expected to be used as a modulator for ultra high-speed and long-distance communications. As an example of a semiconductor Mach-Zehnder modulator, there is disclosed in non-patent literature 1 a lumped optical modulator having a p-i-n structure. In the optical modulator disclosed in the non-patent literature 1, since it has the p-i-n structure and therefore a leak current is small, it is possible to effectively apply an electric field to a core layer.
In addition, there is disclosed a modulator using a Schottky electrode in non-patent literature 2. In the modulator disclosed in the non-patent literature 2, a wider electrical bandwidth is achieved by using a traveling-wave electrode structure as an electrode structure. In addition, in the non-patent literature 1, compared with these, a semiconductor Mach-Zehnder modulator with an n-i-n structure is discussed with a view to further achieving a lower voltage, miniaturization, and a higher speed.
Patent literature 1: WO 2004/081638 pamphlet: Non-patent literature 1: C. Rolland et al., “10 Gbit/s, 1.56 μm multiquantum well InP/InGaAsP Mach-Zehnder optical modulator,” Electron, Lett., vol. 29, no. 5, pp. 471-472, 1993.
Non-patent literature 2: R. Spickerman et al., “GaAs/AlGaAs electro-optic modulator with bandwidth >40 GHz,” Electron, Lett., vol. 31, no. 11, pp. 915-916, 1995.